What Causes My Toddler S Persistent Cough And Recurring Fever After Pneumonia Treatment?

Hi , Iam in US and my son ( 2 years 7 months ) was diagnosed with pneumonia 4 weeks back and even though he had completed his antibiotic course for 10 days , he felt fine but again got cough and fever couple of times already since then and doctor says he seems to have catch another viral infection. .His cough hasn’t gone completely and fever comes back as well .Do you know what’s causing this ? Thanks